Phase modulation has been in practical use as one of the techniques to transmit signals in an optical transmission system. In the phase modulation, data is transmitted by controlling the phase of a carrier wave in accordance with the transmission data. For example, in quadrature phase-shift keying (QPSK) modulation, “θ”, “θ+π/2,” “θ+π” and “θ+3π/2” are assigned respectively to each symbol “00,” “10”, “11” and “01”, each of which consists of 2-bit data. Here, “θ” is an arbitrary phase. A receiver device reproduces transmission data by detecting the phase of the received signals.
Differential quadrature phase-shift keying (DQPSK) is known as a technique to relatively easily implement a QPSK receiver. In the DQPSK modulation, the difference between two successive symbols is assigned with the corresponding phase (0, π/2, π, 3π/2). Accordingly, the receiver device may reproduce the transmission data by detecting the phase difference between the two successive symbols.
FIG. 17 illustrates a related art QPSK transmitter. Here, only a configuration necessary for the description of the operation principle will be illustrated. A QPSK optical modulator 900 is a Mach-Zehnder modulator, which is provided with an optical continuous wave (CW) generated by an optical source (not illustrated). The optical CW is split by an optical splitter, and is guided to a first arm and a second arm. The first arm is provided with a modulator 901 and the second arm is provided with a modulator 902. Signals which passing through the modulator 901 are provided with delays T1 and T2 by a driver 910. Signals which passing through the modulator 902 are provided with delays T3 and T4 by the driver 910. Here, the signals input to the modulator 901 are referred to as an x-polarized wave and the signals input to the modulator 902 are referred to as a y-polarized wave. FIG. 18 illustrates the x-polarized wave and the y-polarized wave. In FIG. 18, a wave 921 is the x-polarized wave and a wave 922 is the y-polarized wave. As illustrated in FIG. 18, the x-polarized wave and the y-polarized waves are two optical waves with mutually orthogonal polarization states. A set of optical signals output from the modulators 901 and 902 are combined to generate DQPSK signals which are the polarization multiplex signals.
In such an optical transmitter which processes a plurality of signals in a multiplexed manner, it is important to precisely control the delay differences among the multiplex signals. Here, the phase difference between the data streams of the x-polarized wave and the y-polarized wave is defined in two ways in the transmission scheme thereof. In the following description, the phase difference between the data streams of the x-polarized wave and the y-polarized wave may be simply referred to as the “phase difference between the x-polarized wave and the y-polarized wave.” Here, the phase difference between the x-polarized wave and the y-polarized wave is represented by, for example, ΔT in FIG. 18. In one of the phase difference types, the data streams of the x-polarized wave and the y-polarized wave are in the same phase, i.e., are in an “aligned” state. In the aligned state, the data streams of the x-polarized wave and the y-polarized wave are provided with no phase difference (i.e., a delay difference). In the other of the phase difference types, the data streams of the x-polarized wave and the y-polarized wave are provided with half a bit of the phase difference (i.e., the delay difference), i.e., are in an “interleaved” state. The phase difference ΔT in FIG. 18 represents the interleaved state. That is, in the optical transmitter, it is important to precisely adjust the phase difference between the x-polarized wave and the y-polarized wave in the aligned or the interleaved state. Since whether to select the aligned or the interleaved state is determined in accordance with a system request, it is preferable that the signals are transmittable in either transmission scheme, i.e., in the aligned state or the interleaved state.
In this regard, a related art technique to perform a phase adjustment of the x-polarized wave and the y-polarized wave with a combination of a fixed delay modulator, a driver which provides a fixed delay, and a phase shifter which provides a phase difference in a preceding stage of the fixed delay modulator has been proposed. Another technique for the retiming of each data string with synchronized clocks has also been proposed. A further technique to perform delay control by providing a variable delay circuit for each data string has been proposed. A further technique to perform a phase adjustment with a variable operation circuit provided for each data string and a differential amplifier provided in the final stage has also been proposed.
The following is a reference document.    [Document 1]: Japanese Laid-Open Patent Publication No. 2006-270909